1. Field of the Invention
The present invention relates to a semiconductor package with a high-frequency element, and more particularly, to a semiconductor package with a cavity formed in the encapsulant corresponding to a high-frequency element thereof so as to avoid the influence of the encapsulant upon the high-frequency element.
2. Description of the Related Art
The package has four major functions, i.e. signal distribution, power distribution, heat dissipation, and protection. In general, the semiconductor chip is formed into an enclosure, such as a single-chip module (SCM) or a chip carrier, referred to as a first-level package, i.e. the packaging of the semiconductor. These packaged chips, along with other components such as capacitors, resistors, inductors, filters, switches, and optical and RF components, are assembled to a printed wiring board in a second-level package.
In the prior art, there exist varied packages, such as a small outline package (SOP), a quad flat package (QFP), a ball grid array (BGA) package, and so on.
Referring to FIG. 1, it depicts a typical ball grid array package 10. The package 10 generally has a substrate 14 and a chip 18 attached on the substrate 14 by an adhesive 28. The chip 18 is electrically connected to the substrate 14 through a plurality of bonding wires 16. An encapsulant 12 encapsulates the substrate 14, the chip 18, and the bonding wires 16. The substrate 14 further has a plurality of solder balls 24 for electrically connecting an external circuit.
The active surface 26 of the chip 18 typically directly touches the encapsulant 12. However, in a specific case, the chip 18 is provided with a high-frequency circuit or element disposed in a high-frequency area 20 on the active surface 26. The signal propagation speed Vp in the high-frequency element is derived from the following equation:Vp=C/√(∈r)
where C is the speed of light in vacuum and ∈r is effective or equivalent dielectric constant. As the high-frequency element is exposed to air, the effective dielectric constant ranges from 1 (the dielectric constant of air) to 4 (the dielectric constant of G-10 or FR-4 substrate which is constructed from a woven glass fabric with an epoxy resin binder). However, while the high-frequency area 20 is covered with the encapsulant 12, the dielectric constant of the encapsulant 12 is larger than that of the air and thus the effective dielectric constant ranges from the dielectric constant of the encapsulant 12 to the dielectric constant of the substrate 14. Therefore, the effective dielectric constant of the substrate covered with the encapsulant 12 is larger than that of the substrate exposed to the air. Furthermore, the loss factor or loss tangent of the high-frequency circuit is also increased because of the covering of the encapsulant 12.
Accordingly, there exists a need for a semiconductor package which is provide with a cavity to prevent the high-frequency element from contacting the encapsulant and avoid the disadvantages due to the contact.